Composite materials lend themselves to bonded structures better than to fastened ones. Bonded composites have received limited use in critical aerospace structures, however, because the bonds can vary in strength or stiffness even if they have no discrete bond line defects (disbonds, porosity, voids, cracking, etc.). Traditional nondestructive inspection methods rely upon quantifying these defects to predict the flightworthiness of the structure, but are unable to ascertain the cohesiveness of the bond at any location if defects are absent. Nondestructive identification of low strength bonds and regions of "kissing unbonds" (bonds of near zero strength) remains a significant goal solved only in a few specific bonded applications where the results of shear or tensile tests have been correlated to a particular NDE signal feature. Modified pulse-echo ultrasonic testing (UT) has been successful in finding the discrete defects (voids, delaminations, porosity), but not "kissing unbonds" and low strength bonds. Infrared thermography, shearography, eddy current, and various high and low frequency ultrasonic methods have also been unsuccessful in discerning bond quality in thermoplastic welds.
The present invention provides a nondestructive method for testing bond quality using an electromagnetic (EM) pulse to induce vibrations in the embedded susceptor and an acoustic receiver to listen to and to record the induced vibrations. Analysis of the received vibration signal discriminates bond quality.